1. Field of the Invention
The present invention relates to a fly back transformer (to be called "FBT" below) which generates a high voltage to be supplied to a cathode ray tube of a TV or a monitor. Particularly, the present invention relates to a focus volume of an FBT and its coupling device, in which a volume board of a focus unit of the FBT is provided in a small size, but its voltage breakdown resisting strength is reinforced. More specifically, the present invention relates to a focus volume of an FBT and its coupling device, in which a resistor pattern having at least one or more round portions with a moving variable contact is formed on a volume board, and the volume board is installed on the focus unit of the FBT. Further, the volume board is open, and the opening covers the center of a virtual circle extending along the round portion of the resistor pattern and also covers a part of a virtual straight line connecting the starting point and the ending point of the round portion. Therefore, when manufacturing the focus volume of the focus unit (which is installed on a side of the FBT case), the focus ceramic board can be separately manufactured, and then, the volume resistor pattern can be printed. Thus the volume board is provided in a small size, but its voltage resisting strength is reinforced. Further, the manufacturing process for the volume board is simplified, and the epoxy resin needs not be filled. Therefore, the volume board can be manufactured in an easy manner. Further, one ceramic board can serve as a small and large volume board, and therefore, the volume board can be standardized.
2. Description of the Preferred Embodiment
In the generally known conventional FBT, high and low voltage bobbins which are installed within the FBT case generate a high voltage to supply it through an anode cable to a cathode ray tube of TV or monitor. Here, the focus unit which is installed on a side of the FBT case varies the focus and screen voltages.
FIG. 1 illustrates the conventional focus unit of the FBT. Referring to FIG. 1, a focus volume knob 30 is integrally formed with a rotating piece 40 which extends into a main body 20 of a focus unit 10. Upon the rotating piece 40, there is installed a slider 50. Upon the slider 50, there is installed a ceramic board 70 on which a resistor pattern 60 having a variable contact and a central contact is printed, the variable contact and the central contact being connected to the contacts of the slider 50. A soft insulating resin 21 is coated upon the ceramic board 70 which has the resistor pattern.
In this conventional focus unit 10 constituted as described above, in order to step up or down the focus and screen voltages, if the focus volume knob 30 (extending to the outside of the main body 20) is manually turned, the slide 50 together with the rotating piece 40 is rotated, so that the voltages can be stepped up or down along the resistor patterns 60 of the ceramic board 70.
Meanwhile, FIG. 2 illustrates the ceramic board (volume board) for use in the conventional focus unit 10. Referring to this drawing, in the resistor pattern 60, a contact point 61 of the slider 50 lies between a shaft 63 of an output focus voltage 62 and the round resistor pattern 60. The contact point of the slider 50 rotates around the shaft 63 and along the contact point 61.
Under this condition, if a high voltage of about 5-10 KV is inputted into an input terminal 67, the high voltage is supplied to between the input terminal 67 and a ground 66, while an output focus voltage 64 is transmitted through the contact point 61 which rotates around the shaft 63. An output screen voltage 65 also is related to a contact point 61' and a shaft 69 in the same manner, and a stepped-down voltage is transmitted through the contact point 61'.
However, owing to the contact point 61 sweeping along the round resistor pattern 60, there is generated a potential difference between the round resistor and the output focus 64. The distance between the resistor of the output focus 64 and the round resistor pattern 60 becomes an important factor for the voltage breakdown resisting strength.
Particularly, the size of the round resistor pattern 60 cannot be expanded within the limited size of the ceramic board 70, and therefore, the voltage breakdown resisting strength has to be increased. Thus conventionally as shown in FIG. 3a, an opening 80 is formed, or as shown in FIG. 3b, a closed opening 90 and/or an opening 80 is formed, thereby reinforcing the voltage breakdown resisting strength.
FIGS. 3a to 3e illustrate various examples of the conventional volume board. FIG. 3a illustrates a volume board having no central point P but having an opening 80. In FIG. 3b, a central point P and a closed opening 90 are formed. In FIG. 3c, the central point P serves as a closed opening, and an opening 80 is formed.
FIG. 3d illustrates a volume board having a closed opening 90. FIG. 3e illustrates a volume board in which two openings 80 covering no central point P are formed near the both ends, and closed openings 90 are formed each at the middle and near the upper end. In the above examples, a sufficient voltage breakdown resisting strength cannot be obtained compared with the provision of the openings from the central point to the edge.
Further, in the case where the above ceramic board 70 having the resistor pattern 60 is used, the inside of the focus unit 10 has to be coated with a soft epoxy resin, and the curing of the resin has to be carried out. Therefore, due to the complicated structure and the complicated manufacturing process, the assemblability and the productivity are extremely aggravated.